The purpose of this competitive renewal application is to continue our ongoing studies on hepatocyte nuclear factor-1 (HNF-1) and its roles in the regulation of kidney-specific gene expression and cystic kidney diseases. HNF-1 is a DNA-binding transcription factor that regulates tissue-specific gene expression in the kidney and other epithelial organs. Mutations of HNF-1 produce renal developmental abnormalities including renal agenesis, hypoplasia, cystic dysplasia, and glomerulocystic kidney disease. To unravel the pathogenesis of these anomalies, we have produced mutant mice that develop phenotypes similar to affected humans. Kidney-specific inactivation of HNF-1 or expression of dominant-negative mutant HNF-1 in transgenic mice produces kidney cysts and renal failure. Analysis of these mutant mice has revealed that HNF-1 regulates the transcription of genes encoding ciliary proteins that are involved in human cystic kidney diseases. These findings demonstrate that HNF-1 plays a central role in cystic and developmental diseases of the kidney. To further elucidate the functions of HNF-1, the proposed studies have two specific aims. The first aim is to understand how HNF-1 regulates Wnt signaling. Preliminary studies have shown that HNF-1 regulates genes that are involved in Wnt signaling, and inhibition of HNF-1 results in abnormal activation of the Wnt pathway, which may underlie cyst formation. Studies will be performed in mutant mice, cultured cells, and organ culture to elucidate the mechanism of activation of Wnt signaling. The second aim is to characterize microRNAs that are regulated by HNF-1 in the kidney. In addition to protein-coding genes, HNF-1 regulates the expression of microRNAs, which are small non-coding RNAs that regulate post-transcriptional gene expression. These findings have revealed a novel pathway by which HNF-1 regulates tissue-specific gene expression and kidney development. The functions of two families of microRNAs that are regulated by HNF-1 will be studied, and their target mRNA transcripts will be identified. Collectively, the proposed studies will advance our understanding of normal kidney development, unravel how mutations of HNF-1 produce kidney diseases, and identify potential therapeutic targets for cystic and developmental diseases of the kidney.